Conventional circulating hot gas dryers, hereinafter conventional dryers, are used to dry various polymers. Some disadvantages of these conventional dryers are (1) long residence times in order to dry the polymers, (2) the use of large vessels to hold the polymer which is being dried, (3) the use of large flows of hot, dry gas, (4) large amounts of energy to run the blower in order to move the gases within the system, and (5) difficulty in controlling moisture of the discharged polymer at the discharge to a fixed moisture level.
More specifically, conventional dryers require long residence times for polyamide resins, because the dryers are usually operated at low temperatures in order to minimize yellowing of the polyamide. Thus, because the temperature is low, usually less than 100.degree. C., a longer residence time is required. This residence time can approach several hours. Moreover, because conventional dryers have long residence times, a large vessel is required to hold the polymer. These large vessels are needed because as the polymer is being dried, which requires a significant amount of time, a vessel is needed to contain the polymer while drying occurs. A large facility is required to house the large vessel.
Furthermore, conventional dryers usually heat the polymer up to the drying temperature by means of a large volume of heated gas flowing through the polymer. Thus, a large flow of hot, dry gas is needed to flow through the vapor space around the polymer. Consequently, large systems to move the gas within the system, i.e. a blower, are needed, thus, more energy is used, approximately 130-150 kW for a 2274 kg/hr dryer, to run the blower to generate this flow.
Lastly, conventional dryers, due to their long residence time, are difficult to control in such a way that the discharge moisture is maintained at a fixed value. Specifically, changes made to the dryers during the operation of a process take a long time to materialize due to the amount of time the polymer is in the dryer. For example, if a change is made in the processing system, full effects of that change will not show up in some cases for up to 24 hours.
The present invention solves the problems of conventional dryers by using high frequency electric fields to dry polyamides with low temperatures, short times, and less investment to achieve a high quality polyamide resin with low moisture.
In particular, the process of the present invention can dry thermoplastic polyamide resins very quickly, in less than 15 minutes, with no increase in yellowness. The drying for polyamides can be achieved at low temperatures. In general, high frequency electric fields speed up the diffusion of water in the polyamide resins by disrupting the hydrogen bonds between the water and the amide links. This allows the water to diffuse out of the polyamide resins very quickly, consequently significantly reducing the residence time. In addition, minimizing the exposure of polyamide resin to oxygen during the drying process, at the reduced residence time and low temperature eliminates or minimizes the increase in yellowness during drying. Moreover, due to the short residence times a small vessel is used to the hold the thermoplastic polyamide resin; thus, less space is required to house the process of the present invention. In addition, heating with high frequency electric fields eliminates the need for large flows of hot, dry gas. More specifically, high frequency electric fields heat the polyamide resin directly; therefore an inert gas is used only to take the water away from the polyamide resin. Thus, there is less gas flow per unit volume of polymer. Furthermore, as a result of the direct heating of the polyamide resin less energy is used with the process of the present invention to circulate the gas. More specifically, due the shallow bed of the polyamide resin (resulting from the short residence time) and the very low flow of the gas, approximately 7.5 kW for a 2275 kg/hr dryer to run the blower, a lot less energy is required for high frequency electric field drying than for conventional dryers.
High frequency electric field drying is easier to control than conventional drying to a fixed polymer discharge moisture due to the short residence time. More specifically, full effect of changes made in the process of the present invention can be seen in less than 15 minutes.
It is known in the art that polyamides may be dried by the use of microwaves. Polyamide dryers using microwaves are prone to hot spots because the microwave wavelength is usually smaller than the size of the cavity in which the resin is being heated. Due to the presence of unavoidable standing waves in microwave cavities, there are areas in those cavities with peaks and valleys in the level of electric field. Various methods are included to attempt to minimize these variations in heating of polyamide resin, such as `mode stirrers` to continuously vary the location of these field variations, and such as mechanically agitators to continuously move the resin. These methods are not altogether effective, as the mode stirrer cannot create truly uniform heating due to the repeating pattern of the rotating vanes or antennas from the microwave source, and mechanical agitation causes abrasion in the resin which then creates fines and dust.
By contrast, the high frequency electrical energy in the present invention has a wavelength which is longer than the thickness of the bed of resin being processed, and is typically longer than even one axis of the electrode structure, such as the width; thus, the electric fields established by that energy and the heating produced by such fields are more uniform than that of microwaves. More specifically, the wavelength of a 2450 MHz microwave is about 11 cm, as compared to 4 meters for a 75 MHz high frequency electrical energy.
Moreover, microwave radiation is limited in penetration depth in high dielectric loss materials, and is thus attenuated as it enters a large volume of polyamide resin, resulting in lower heating rates in the interior. This limits the size of microwave dryers to batches of materials which are smaller than the capability of the present invention. Optimally, the high frequency electric field is perpendicular to the thickness of resin, and all materials is exposed to the same voltage gradient through the depth of the bed from hot electrode to ground electrode.
In addition, for commercial scale operation, microwave dryers for polyamide resins require residence times of about 90 minutes. Although, this is a shorter residence time than required by conventional dryers, it is still a long residence time.